webdriver: Refactor webdriver session and improve window handles (#38745)

This PR contains 2 parts:

1. Refactor webdriver session.
2. Improve webdriver window handles:
- Webdriver always get window handles from script thread by default.
- If script thread is blocked by user prompt, embedder stores the window
handle before user prompt appears, then webdriver can get window handle
from embedder.

Testing: Clear timeout cause by user prompt blocking script thread:
https://github.com/longvatrong111/servo/actions/runs/17033900026

---------

Signed-off-by: batu_hoang <hoang.binh.trong@huawei.com>
Signed-off-by: batu_hoang <longvatrong111@gmail.com>
Co-authored-by: Euclid Ye <euclid.ye@huawei.com>

components/script/script_thread.rs

@@ -2436,9 +2436,6 @@ impl ScriptThread {
             WebDriverScriptCommand::RemoveLoadStatusSender(_) => {
                 webdriver_handlers::handle_remove_load_status_sender(&documents, pipeline_id)
             },
-            WebDriverScriptCommand::GetWindowHandle(reply) => {
-                webdriver_handlers::handle_get_window_handle(pipeline_id, reply)
-            },
             _ => (),
         }
     }